Process and apparatus for removing volatile substances from viscous compositions

ABSTRACT

A process for removing volatile substances from viscous compositions, for example, removing volatile solvent and unreacted monomer from polystyrene. The composition is heated to a temperature above the boiling points of the volatile substances at the pressure employed in the operation. The composition is then formed into strands and the strands are impacted against a solid surface. The composition is then caused to flow as a film over a heated surface while the strands and film are exposed to sub-atmospheric pressure.

United States Patent Kimoto et al.

[541 PROCESS AND APPARATUS FOR REMOVING VOLATILE SUBSTANCES FROM VISCOUSCOMPOSITIONS Inventors: Koji Kimoto, Kamakura, Kanagawa- Prefl;Yoshinari Yamagisawa, Yokohama, Kanagawa-Prefl, both of Japan ToyoKoatsu Industries, Incorporated, Tokyo, Japan Filed: Oct. 15, 1970 Appl.No.: 90,233

Related US. Application Data Continuation of Ser. No. 765,890,0ct. 8,1968, abandoned.

Assignee:

References Cited UNITED STATES PATENTS 2,849,430 8/1958 Amos et al..159/2 E UX [451 Sept. 26, 1972 3,351,119 11/1967 Rosenblad ..l59/l 3 B3,395,746 8/1968 Szabo et a1 ..264/102 UX 3,410,938 11/1968 Schippers..264/102 Primary ExaminerRobert F. White Assistant Examiner-J. l-l.Silbaugh Altorney--Christen and Sabol [57] ABSTRACT strands and film areexposed to sub-atmospheric pressure.

7 Claims, 3 Drawing Figures PROCESS AND APPARATUS FOR REMOVING VOLATILESUBSTANCES FROM VISCOUS COMPOSITIONS This application is a continuationof Ser. No. 765,890 filed Oct. 8, 1968 and now abandoned.

BACKGROUND OF TI-IEINVENTION 1. Field of the Invention This inventionrelates to a process for removing volatile substances, such as unreactedmonomers and/or solvents, from viscous compositions such as polymers ofstyrene and/or its homologs.

2. Description of the Prior Art Usually, polymer compositions producedby bulk polymerization or solution polymerization necessarily containvolatile substances such as unreacted monomer or solvent, invaryingamounts depending on the particular conditions of thepolymerization process. In using such polymer compositions for injectionmolding or extrusion molding, it is necessary to remove the volatiles tovery low residual amounts.

Especially when styrene polymer compositions are produced by the aboveprocesses, the residuals should be kept usually below about 0.5 wt.percentto obtain sound moldings devoid of molding defects and havingexcellent practical physical properties, such as, heatdistortiontemperature, weatherproofness, etc.

Generally, many methods have been proposed heretofore for removingvolatile substances from such polymer compositions in viscous liquidform, for example,

1. There is the type in which the polymer structure is divided into finestreams or stands under vacuum to expose a large surface area to themass, and

2. The type in which the liquid polymer composition under vacuum flowsdown as a film along the wall of a vessel.

However, in type (1), due to the latent heat of evaporation of thevolatile substances, the surface of the liquid polymer composition coolsrapidly and the viscosity of the surface of the strand increases quicklyas the evaporation progresses. Thus, the evaporation can reach a certainlimit beyond which it cannot proceed.

In type (2), especially when such large amounts as 40 to 60 wt. percentof volatiles are present in the liquid polymer composition, foamingoccurs because of the evaporation of the volatiles and the film becomesbulky giving no effective flow-down of the film.

When small amounts such as l to wt. percent of volatiles are containedor at the stage where the amount is reduced by reduced to such anextent, the liquid becomes so highly viscous that it becomes verydifficult to obtain an effective flow-down on the wall or the polymercomposition peels down off the wall.

Accordingly, the above two methods are not practical and areinsufficient to remove volatile substances fully set styrene polymercompositions containing them to the extent that the residual volatilesmay be less than 0.5 wt. percent as required to obtain sound moldings.Usually, therefore, a complicated operation of two steps is employed byusing vented extruders or similar devices in connection with theabovementioned methods.

SUMMARY OF THE INVENTION The present invention provides a process forremoving volatile substances from styrene polymer compositions producedby bulk polymerization or solution polymerization, wherein the residualvolatile substances can be reduced in one operation to a maximum ofabout 0.5 wt. percent without the disadvantages of the prior processesas mentioned above.

The process of the present invention comprises (1) heating thecomposition to a temperature above the boiling points of the volatilesubstances contained in it but below the temperature at which thepolymer composition is thermally denaturalized, pressurizing anddischarging the composition in strand form through an orifice into avacuum system maintained below an absolute pressure of mm.Hg therebyevaporating and separating some of the volatile substances, (2) in thesame vessel, subjecting the strand to impact by collision with a heatedsurface positioned below the orifice after discharge from the orifice,and (3) causing the composition after impact to flow downgravitationally along the heated surface while transferring heat throughthe heated surface to the composition thus compensating for a portion ofthe latent heat of evaporation consumed in the evaporation of thevolatile substances and whereby, at the same time, the surface of thecomposi tion exposed to the vacuum is renewed while flowing down so asto accelerate the vaporization of the volatile substances.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a vertical section through devolatilizing apparatusillustrating one embodiment of the invention;

FIG. 2 is a horizontal cross-section through the devolatilizingapparatus, taken along line 2-2 of FIG. 1; and

FIG. 3 is an enlarged vertical section through the strand former of theapparatus shown in FIG. 1 taken on line 3--3 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT One embodiment of an apparatusthe present invention is shown in FIGS. 1, 2, and 3 of the attacheddrawings. In this embodiment, polymer feed pipe 1 is connected to theinterior of toroid-shaped strand former 2 positioned in the upperportion of jacketed vacuum tank 3. The strand former 2 is provided witha polymer distributor head 4 having a plurality of downwardly directedfine orifices 5 through which the polymer is forced to form strands 6.The strand former 2 is also provided with a heating jacket 7 on itsupper parts. The jacket 7 is divided by rib 8 into an outer conduit 9and an inner conduit 10. The rib 8 does not extend all the way aroundand, thus, the conduits 9 and 10 are interconnected through gap 11. Theouter conduit 9 is connected to pipe 12 through which heating fluid isfed and conduit 10 is connected to pipe 13 through which the heatingfluid exits.

The vacuum tank 3 comprises a shell 14 having an opening 15 for reducingthe pressure within the tank and for withdrawing volatile substances andan outlet 16 at the bottom for taking out the devolatilized polymercomposition. The tank 3 is also provided with a heating jacket 17 havingan inlet 18 and an outlet 19 for circulation of heating fluid.

Internally mounted within the shell 14 is an elongated toroid-shapedheater 20. The upper surface of heater 20 is formed with peaked roof 21.The inner walls 22 of heater 20 are tapered downwardly towards eachother and the outer walls 23 are tapered downwardly away from each othersuch that the heater is wider both in cross-section and in overall widthat the bottom and narrower at the top. The heater 20 is so arrangedwithin shell 14 that the strands 6 fall upon the peaked roof 21 as bestshown in FIG. 1. If desired, the walls 22 and/or 23 may be vertical ornearly so, the important factor being that the polymer composition iscaused to flow downwardly in the form of a film. The heater 21 is hollowand is provided with heating fluid inlet 24 and outlet 25 forcirculation of heating fluid within it.

In operation, the polymer composition to be treated, comprising astyrene polymer produced by bulk polymerization or solutionpolymerization and volatile substances such as solvent and unreactedmonomer, is heated to a temperature above the melting point of thepolymer and above the boiling points of the volatile substances at thepressure within shell 14 but below the temperature at which the polymercomposition begins to thermally decompose. The heated composition is fedthrough pipe 1 into distributor head 4 where it flows under pressure tothe fine orifices 5. The composition then is injected into the vacuumtank 3 through the fine orifices to form strands.

The efficiency of evaporation of the volatile substances with respect toeffective inner space of the shell 14 may be made higher if the formedstrands do not I contact each other or with the inner wall of shell 14.

The size and shape of the fine orifices 5 are suitably chosen accordingto the vapor pressure of the volatile substances in the heated polymercomposition and the viscosity of the heated polymer composition so thatcontinuous strands are readily formed even though foaming takes place inpassing through the orifices. The pressure usually employed in the feedpipe 1 and distributor head 4 is in the range of about 2 to about 20kglcm for example, when the orifices 5 are circular and have a diameterof about 3 mm.

Thus, by dividing the liquid into strands in a foaming state, thevolatiles not only in the surface part of the strand itself but also inits inner part are effectively evaporated and separated from the polymerbeing treated. However, since the latent heat of evaporation of thevolatile substances is supplied by the strands 6 as they traveldownwardly, their own temperature drops, and the viscosity of theirsurfaces rises.

Consequently, the diffusion velocity of the volatile substances in thestrands 6 drops quickly. Even if the dropping distance of the strands 6is increased, no substantial evaporation continues and the desiredobjects cannot be attained.

We have found that effective evaporation of the volatile substances isaccelerated first by the collision of the strands 6 with a solid surfacein the same tank, that is, with the peaked roof 21 of the heater 20.

The high viscosity surface layer of the strand is broken by the impactaccompanying this collision, and

the concentration gradient of the volatile substances present,respectively, in the surface and inner parts of the strands iscorrected.

Secondly, the evaporation is also markedly accelerated bygravitationally flowing the polymer composition after impact, downwardlyon the walls 22 and 23 of the heater 20. In this case, the liquidreceives a supply of a heat quantity from the heater 20 to compensatethe latent heat consumed in the evaporation of the volatile substancesand flows down in the form of a film exposing a large surface area tothe vacuum. As a result, vapor pressure of the residual volatilesubstances increases and the viscosity of the composition decreases.While flowing down, parts of the surface of the film exposed to thevacuum are continually replaced with parts in contact with the walls ofthe heater 20. Thus, the concentration gradient between the surface andthe inside of the film is continuously corrected and the evaporation isaccelerated.

After the above treatment, residual volatile substances in the polymercomposition are less than about 0.5 wt. percent and the composition isaccumulated in the bottom part of the shell 14 of vacuum tank 3 andgradually taken out through the outlet 16.

The volatile substances evaporated and separated from the treated liquidin shell 14 can be easily taken out through the opening 15 by means ofany suitable vacuum system, recovered and recycled to the polymerizationsection.

The main point of the present invention is that in removing volatilesfrom the highly viscous liquid mixture (in this embodiment, the polymercomposition) three important factors, i.e., ejection of liquid mixturein the form of strands into the vacuum tank 3 and collision of thestrands with the roof 21 of the heater 20 in the same vesselsuccessively and flowing the mixture gravitationally downwardly alongthe walls 22 and 23 of the heater 20 in the vacuum tank, are combinedand worked in this order consecutively in the same vacuum tank 3.

The influence of the degree of vacuum on the amount of residual volatilesubstances is large. The degree of vacuum required depends usually onthe kind of components of the liquid to be treated and the ranges oftemperature applied. The lower pressure gives the better evaporation,but the lower limit is practically determined by the condensationtemperature of volatiles to be recovered in recovery condensersconnected with opening 15. For the above polymer composition, thepressure in the tank 3 usually used is about 50 to about mm.Hg.,absolute.

The temperature of the liquid to be treated is preferably above theboiling points of the volatile substances for practical use, but below atemperature at which the influence of thermal decomposition begins toappear. When treating the above polymer composition, the range fromabout C. to about 300C. is suitable.

The temperature of the heater 20 also is suitably maintained above theboiling points of the volatiles but below the temperature ofdecomposition of the viscous liquid; for example, temperatures of about150C. to 250C. are suitable.

Practically speaking, the diameter of orifices 5 is preferably below 4mm. for forming strands such as are described above. But, if the orificediameter is less than 1 mm., the orificetends to become clogged byforeign substances, such as, pieces of gasket, dust, etc., and thereforeis not recommendable for industrial use.

The process for removing volatile substances according to the presentinvention can be applied to many other compositions and can be readilyapplied particularly to thermoplastic polymer compositions containingstyrene, or substituted styrene as a component, such as polystyrenes forgeneral purposes, impact resistant polystyren'es, acrylonitrile-styrenecopolymerized resins, acrylonitrile-butadiene-styrene copolymerizedresins and modified acrylonitrile-butadiene-styrene type resins.

Furthermore, in working the present invention, the form of the feedingpipe, distributor head and orifices, the manner of arrangement of theorifices, the form and arrangement of the heater and other structuralfactors may be varied and modified in many ways other than are shown inthe attached drawings.

The following examples are presented.

EXAMPLE 1 A reaction solution obtained by the bulk polymerization ofstyrene and containing wt. percent ethyl benzene, 8 wt. percent styreneand 82 wt. percent polystyrene was heated to about 250C. to about 260C.,and was injected continuously in the form of strands through fourorifices each having a diameter of 3 mm. using a pressure of 4 to 13kg/cm into a vacuum tank 3 maintained under a vacuum of 30 to 60mm.l-lg, absolute. The strands were made, consecutively in the sametank, to collide with the roof of a heater which had a conical roofhaving an apex angle of 45 and downwardly expanded side walls which wereeach inclined by 3 with respect to the axis of the heater. The heaterwas maintained at 260C. to 270C. and was set about 200 mm. below theorifices. After impact, the composition in the form of a film was madeto flow down the side walls for 400 to 800 mm. gravitationally. Thetemperature of the polystyrene accumulated in the bottom part of thevacuum tank 3 was about 230C. to 240C. which was about 10C. to 15C.higher than the value calculated from the latent heat of evaporation ofthe volatilized volatile substances. The polymer composition was takenout and was analyzed by gas-chromatography. The result of analysisshowed that the residual volatile substances content in the polymercomposition treated under these conditions was 0.1 to 0.3 wt. percent.In comparison, when the liquid was treated under the same conditionsexcept that the roofshaped heater was removed, the residual volatilecontent was found to be 2 to 4 wt. percent.

EXAMPLE 2 ol mer com osition ccu ul d in the lower art of Fhe vacuumtzihk was a ow 238C. After analysis? in the same manner as in Example 1,the residual volatile content was found to be 0.2 to 0.5 wt. percent.

What is claimed is:

1. Process for removing volatile substances including unreacted monomerand solvent from a reaction solution obtained by polymerization of saidmonomer and containing a substantial portion up to 60 weight percent ofsaid volatile substances in admixture with a viscous relativelynon-volatile polymer, which process comprises the steps of:

A. heating said mixture to a temperature above the boiling points ofsaid volatile substances but below the temperatures at which saidnon-volatile polymer begins to thermally decompose;

B. subjecting said mixture to a pressure of from 2 to 20 kg/cm andforming a strand in foaming condition to volatilize substantial portionsof said volatile substances;

C. impacting said strand at sub-atmospheric pressure with a firstsurface to expose and volatilize additional portions of said volatilesubstances; and

D. flowing said mixture after impact in the form of a film over a heatedelongated and descending substantially vertical second surface undersub-atmospheric pressure to volatilize further amounts of said volatilesubstances, the residual amount of said volatile substances being amaximum of about 0.5 weight percent of said mixture.

2. Process as claimed in claim 1 wherein said polymer is polystyrene andsaid monomer is styrene.

3. Process as claimed in claim 2 wherein the temperature of said mixturein steps (A), (B), (C), and (D) is the range of about 150C. to about300C.

4. Process as claimed in claim 3 wherein the subatmospheric pressure insaid steps is in the range of about 50 to about mm./l-lg, absolute.

5. Process as claimed in claim 1 wherein said strand in foamingcondition is formed by discharging said mixture under said pressurethrough an orifice.

6. Process as claimed in claim 5 wherein said orifice is circular havinga diameter of l to 4 mm.

7. Process as claimed in claim 6 wherein a plurality of orifices isprovided.

2. Process as claimed in claim 1 wherein said polymer is polystyrene andsaid monomer is styrene.
 3. Process as claimed in claim 2 wherein thetemperature of said mixture in steps (A), (B), (C), and (D) is the rangeof about 150*C. to about 300*C.
 4. Process as claimed in claim 3 whereinthe subatmospheric pressure in said steps is in the range of about 50 toabout 100 mm./Hg, absolute.
 5. Process as claimed in claim 1 whereinsaid strand in foaming condition is formed by discharging said mixtureunder said pressure through an orifice.
 6. Process as claimed in claim 5wherein said orifice is circular having a diameter of 1 to 4 mm. 7.Process as claimed in claim 6 wherein a plurality of orifices isprovided.